1. Field of the Invention
The present invention relates to a semiconductor memory device. More particularly, the present invention relates to a resistance change memory including a test circuit.
2. Description of the Related Art
As a semiconductor memory device, a resistance change memory using a resistance change element for a memory cell is known. In reading of this resistance change memory, there is a method of determining a resistance state of the memory cell by comparing a read current flowing through the memory cell when a read voltage is applied with a reference current flowing through a reference cell when a reference voltage is applied by a current comparison circuit (a current sense amplifier). Since this method adopts a differential amplification scheme using the reference cell having the same electrical characteristics as those of the memory cell, it has an advantage that an operating range can be expanded by compensating for a change in resistance due to, e.g., a fluctuation in power supply voltage and a fluctuation in temperature. However, since a resistance of the reference cell fluctuates, the reference current also fluctuates.
It is to be noted that a thin-film magnetic substance memory device that can execute reading data by using a dummy cell configured like a normal memory cell has been already proposed in relation to the reading method of the resistance change memory (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-297072).
Meanwhile, in the resistance change memory, there are a bit that a resistance of a resistance change element is abnormally high (an open bit), a bit that a resistance is abnormally small (a short bit), and others as small-number defective bits. These small-number defective bits can be remedied based on substitution using a redundant bit. To realize this remedy, a small-number defective bit must be specified. As a specifying method, there is a method of using a sense amplifier to determine a resistance change element having a resistance smaller than a boundary reference current as a short bit and determine a resistance change element having a resistance larger than the boundary reference current as an open bit. However, in the case of the differential amplification scheme, since the reference current fluctuates, accurate determination of a defective bit is difficult.